This research was funded by the Chinese National 973 Program (2007CB411307), Chinese NSFC Grants (40730315 and 40872122) and Hong Kong RGC grants (7066/07P, 7063/06P and 7053/08P). The final version of the paper has benefited from the perceptive and critical comments of journal reviewers F. Corfu and Shuwen Liu and journal editor Wulf Mueller. This is The Institute for Geoscience Research (TIGeR) Publication No. 210.